SANFORD, Fla. (AP) - One of the most important prosecution witnesses in George Zimmerman's murder trial has left the witness stand.

Rachel Jeantel testified Thursday for a second consecutive day about the last phone call she had with 17-year-old Trayvon Martin in the moments before his deadly encounter with Zimmerman.

She is one of the prosecution's most important witnesses since she bolsters prosecutors' contention that Zimmerman was the aggressor in his confrontation with Martin.

Zimmerman is pleading not guilty to second-degree murder, claiming self-defense.

.George Zimmerman's defense attorney on Thursday challenged the credibility of a key prosecution witness' account of what she heard over the phone in the moments before the deadly exchange between the neighborhood watch volunteer and 17-year-old Trayvon Martin that left the Miami teen with a bullet in his chest.

The defense attorney challenged 19-year-old Rachel Jeantel about her claim that the encounter was racially charged and about her differing accounts of what she heard over the phone when Zimmerman and Martin first exchanged words. The often-testy exchanges between Jeantel and defense attorney Don West came a day after Jeantel admitted to lying about her age and lying about why she skipped Martin's funeral.

Jeantel was on the phone with Martin as he walked from a convenience store through the neighborhood where he was visiting, and she was the last person to speak with him before his encounter with Zimmerman on Feb. 26, 2012 at the Retreat at Twin Lakes townhome complex. Jeantel is one of the prosecution's most important witnesses because she bolsters the contention that Zimmerman was the aggressor.

Jeantel has given slightly differing versions about what she has heard in a deposition, in a letter to Martin's mother and in a recorded interview with an attorney for the Martin family. Among the differences cited by West:

- In some accounts, she said race was an issue but not in others.

- Jeantel testified Wednesday that her friend's last words were "Get off! Get off!" before Martin's phone went silent. But on Thursday, under cross-examination, she conceded that she hadn't mentioned that in her account of what happened to Martin's mother, Sybrina Fulton. She had left out some details to spare Fulton's feelings, and also because neither Fulton nor the Martin family attorney asked her directly about them, Jeantel said.

- After Martin asks why he is being followed, Zimmerman responds, "What are you doing around here?" in one account by Jeantel. In another account, according to West, she says Zimmerman said, "What are you talking about?"

Zimmerman, 29, could get life in prison if convicted of second-degree murder. Zimmerman followed Martin in his truck and called a police dispatch number before he and the teen got into a fight.

Zimmerman has said he opened fire only after the teenager jumped him and began slamming his head against the concrete sidewalk. Zimmerman identifies himself as Hispanic and has denied the confrontation had anything to do with race, as Martin's family and their supporters have claimed.

Jeantel testified Thursday that she thought race was an issue because Martin told her he was being followed by a white man. She told jurors on Wednesday that Martin had described being followed by a "creepy-ass cracker."

"He was being followed," Jeantel said.

But West responded, "It was racial because Trayvon put race in this?"

The exchanges got testier as the day progressed.

When asked by West if she had previously told investigators that she heard what sounded like somebody being hit at the end of her call with Martin, Jeantel said, "Trayvon got hit."

"You don't know that? Do you? You don't know that Trayvon got hit," West answered angrily. "You don't know that Trayvon didn't at that moment take his fists and drive them into George Zimmerman's face."

Later in the morning, West accused Jeantel of not calling police after Martin's phone went dead because she thought it was a fight he had provoked.

"That's why you weren't worried. That's why you didn't do anything because Trayvon Martin started the fight and you knew that," West said.

"No sir!" Jeantel said. "I don't know what you're talking about."

At one point, West handed her a letter she had written with the help of a friend to Martin's mother explaining what happened. She looked at it but then said she couldn't read cursive handwriting.

Jeantel recounted to jurors on Wednesday how Martin told her he was being followed by a man as he walked through the Retreat at Twin Lakes townhome complex on his way back from a convenience store to the home of his father's fiancee.

She testified that Martin described the man following him as "a creepy-ass cracker" and that he thought he had evaded him. But she said Martin told her a short time later the man was still behind him, and she told him to run.

Martin said Zimmerman was behind him and she heard Martin ask: "What are you following me for?"

She then heard what sounded like Martin's phone earpiece dropping into wet grass, and she heard him say, "Get off! Get off!" The phone then went dead, she said.

During Wednesday's testimony, she bristled at West's questions, and at one point she urged West to move on to his next question: "You can go. You can go."

Jeantel's testimony was more subdued on Thursday, at least in the beginning, and West took note of her calmer demeanor. She answered many of West's questions by repeating "yes, sir," almost in a whisper.

"You feeling OK today? You seem different than yesterday," West said.

"I got some sleep," she answered.

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